The present invention relates to an adaptive filter, a control method of an adaptive filter and a storage medium storing therein a program thereof. More to particularly, this invention relates to an adaptive filter which is suitable for use in an echo canceller which is employed in data transmission and in an acoustic system, an automatic equalizer for digital data transmission, or further generally use in identification of unknown system or so forth, and a control method for implementing tap weight control of this adaptive filter, and a storage medium storing therein a program for executing such control method.
Firstly, there is described the principles of adaptive filter.
FIG. 1 is a principle view of the adaptive filter. In FIG. 1, the adaptive filter 100 prepares an estimated value of unknown signal series from a known filter input signal series. The adaptive filter 100 updates a parameter belonging to the filter on the basis of signal series of error between such the estimated value series and the unknown signal series. The adaptive filter 100 identifies unknown system 101 rightly.
Additive noise at the time of ordinary observation is added to the unknown signal series. The adaptive filter 100 converges from initial state of unlearned to final state. Further, in frequent cases, the unknown signal series is given as the answer to the above-described signal series of the unknown system 101. This case is corresponding to the echo canceller and/or the automatic equalizer.
In frequent cases, the adaptive filter is materialized as a non-recursive filter (FIR; Finite Impulse Response). As shown in FIG. 2, the adaptive filter has constitution using delay unit of plural stages. FIG. 2 shows a control circuit of the k-th tap weight. Here, it causes the k-th tap weight ck(n) to be controlled while obtaining correlation value using the sum between an error signal en and an additive noise vn and a filter input signal an-k in such the tap. In FIG. 2, xe2x80x9cnxe2x80x9d is the time, xe2x80x9canxe2x80x9d is an input signal, and xe2x80x9cacxe2x80x9d is a step gain.
Now, a tap weight control algorithm uses a correlation value obtained in such a way that the sum between the error signal and the additive noise is multiplied by the input signal. As the tap weight control algorithm, next LMS algorithm is well known. Further, it is utilized widely.
It can be written as follows about the k-th tap weight:
ck(n+1)=ck(n)+xcex1c(en+vn)an-kxe2x80x83xe2x80x83(1)
Next, another tap weight control algorithm uses a correlation value obtained in such a way that the polarity of the sum between the error signal and the additive noise is multiplied by the polarity of the input signal. As the tap weight control algorithm, next sign-sign algorithm is well known.
It can be written as follows about the k-th tap weight:
ck(n+1)=ck(n)+xcex1csgn(en+vn)sgn(an-k)xe2x80x83xe2x80x83(2)
Here, sgn ( ) is polarity function, thus if factor is positive (or negative), xe2x80x9c+1xe2x80x9d (or xe2x80x9cxe2x88x921xe2x80x9d) is outputted.
The above-described sign-sign algorithm can be materialized very easily because the polarity is multiplied by the polarity. Simultaneously, there is advantage that the operation is stabilized when there is large domestic trouble in the additive noise and the input signal. However, there is the fault that convergence of the filter is slow in comparison with the above described LMS algorithm.
Consequently, it is desired that it causes speedy convergence which the LMS algorithm has to be realized while making the best use of above characteristic of the sign-sign algorithm.
In view of the foregoing, it is an object of the present invention, in order to overcome the above mentioned problem, to provide an adaptive filter, a control method of the adaptive filter and storage medium storing therein a program thereof which realize the adaptive filter and its tap weight control method, and which is fit of domestic trouble after convergence, and which are sign-sign algorithm with high-speed convergence.
According to a first aspect of the present invention, in order to achieve the above mentioned object, there is provided an adaptive filter having non-recursive filter, which acquires a correlation value obtained in such a way that a sum between an error signal and an additive noise is multiplied by a value of input signal in respective taps, before updating weight coefficient of the respective taps according to a product between a value which is obtained in such a way that the correlation value undergoes action of non-linear function and a step gain, the adaptive filter comprises a control means for controlling a power-number of a power function on the basis of an estimated value of electric power of said error signal while taking said non-linear function to be the power function of said correlation value.
According to a second aspect of the present invention, in the first aspect, there is provided an adaptive filter, wherein the control means consists of an estimation means for obtaining an estimated value of the error signal from an average value of the correlation value, and a power generation means for generating the power on the basis of the estimated value.
According to a third aspect of the present invention, in the second aspect, there is provided an adaptive filter, wherein the power-number generation means consists of a comparison means for comparing the estimated value with a threshold value, and a calculation means for calculating power-number which is either the maximum value or xe2x80x9c0xe2x80x9d (zero) from comparison result of the comparison means.
According to a fourth aspect of the present invention, in the second aspect, there is provided an adaptive filter, wherein the power-number generation means consists of a conversion means for converting the estimated value into a decibel value, and a calculation means for calculating a power-number while using the decibel value, a minimum decibel value, and a maximum decibel value.
According to a fifth aspect of the present invention, there is provided a control method of an adaptive filter in which the adaptive filter having non-recursive filter, which acquires a correlation value obtained in such a way that a sum between an error signal and an additive noise is multiplied by a value of input signal in respective taps, before updating weight coefficient of the respective taps according to product between a value which is obtained in such a way that the correlation value undergoes action of non-linear function and a step gain, the adaptive filter comprising the step of controlling a power-number of a power function on the basis of an estimated value of an electric power of the error signal while taking the non-linear function to be the power function of the correlation value.
According to a sixth aspect of the present invention, there is provided a storage medium storing therein a program for executing processing of a control method of an adaptive filter in which the adaptive filter having non-recursive filter, which acquires a correlation value obtained in such a way that a sum between an error signal and an additive noise is multiplied by a value of input signal in respective taps, before updating weight coefficient of the respective taps-according to product between a value which is obtained in such a way that the correlation value undergoes action of non-linear function and a step gain, the adaptive filter comprising the step of controlling a power-number of a power function on the basis of an estimated value of an electric power of the error signal while taking the non-linear function to be the power function of the correlation value.